This application claims the priority of German Application No. 198 14 572.1, filed Apr. 1, 1998, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a process and a braking arrangement for an exhaust gas turbocharger having a variable turbine geometry in which an actuating signal influencing the position of the turbine geometry is determined.
From International Patent document WO 96/39573, a process is known for controlling the charging pressure in an internal-combustion engine charged via an exhaust gas turbocharger having an adjustable turbine geometry. In order to achieve a braking effect in the braking operation of the internal-combustion engine, the turbine geometry is changed into a ram position, in which the flow cross-section of the flow duct is reduced for building-up an exhaust gas counterpressure in the pipe section between the cylinders and the exhaust gas turbocharger. The exhaust gas flows at a high speed through the ducts between the guide blades of the turbine geometry and acts upon the turbine wheel, whereupon the compressor builds up an excess pressure in the intake pipes. As a result, the cylinder is acted upon on the input side by an increased charging pressure. On the output side, an excess pressure exists between the cylinder outlet and the exhaust gas turbocharger. This excess pressure counteracts the blowing-off or releasing of the air compressed in the cylinder by way of decompression valves into the exhaust gas pipes. In the braking operation, during the compression stroke, the piston must carry out compression work against the high excess pressure in the exhaust gas pipes, whereby a high braking effect is achieved.
In the case of the exhaust gas turbocharger known from this document, the braking performance is adjusted to a desired value using a brake regulator. As input quantities of the regulator, the position of the throttle valve in the intake pipes, the rotational engine speed, and the position of the brake pedal are detected, as well as additional quantities representing the operating condition of the vehicle such as the position of an external brake switch or the cruise control adjustment. An actuating signal is generated as an output quantity which acts upon an actuator for adjusting the guide blades of the turbine geometry.
Although this control makes it possible to bring the braking performance as a function of the engine operating temperature to a desired quantity or to a desired course, International Patent Document WO 96/39573 discloses no possibility for generating a maximum braking performance in order to achieve the shortest possible braking distance in order to maintain a constant speed, for example, on down grades.
From German Patent document DE 195 31 871 C1, a process is known for controlling, in the case of an internal-combustion engine charged via an exhaust gas turbocharger with an adjustable turbine geometry, the charging pressure to a defined operating-point-dependent desired value. According to German Patent document 195 31 871 C1, during the transient operation, particularly after a positive load change from low load and rotational speed ranges, control of the internal-combustion engine is improved by simple devices in that the difference between the exhaust gas counterpressure and the charging pressure is supplied to the control as an input quantity. However, German Patent document DE 195 31 871 C1 shows neither an engine braking operation for turbines having a variable turbine geometry in general, nor does this document disclose maximizing the engine braking performance.
The invention is based on the problem of providing a braking performance maximum for all rotational speed ranges of the engine.
According to the invention, this problem is solved by a process and braking arrangement for adjusting an exhaust gas turbocharger having a variable turbine geometry in the engine braking operation. In one embodiment, the charging pressure (p.sub.2S) in the intake pipe and the exhaust gas counterpressure (p.sub.3) in the exhaust gas pipes are measured as engine operating parameters in front of the exhaust gas turbocharger. The total pressure (p.sub.total) is determined as the sum of the charging pressure (p.sub.2S) and the exhaust gas counterpressure. The turbine geometry is adjusted to such an extent that the total pressure (p.sub.total) corresponds to a maximal pressure (p.sub.max).
In another embodiment, the rotational engine speed is measured as the engine operating parameter and a defined position of the turbine geometry is assigned to each rotational engine speed (n.sub.engine), in which position, the total pressure (p.sub.total), as the sum of the charging pressure (p.sub.2S) and of the exhaust gas counterpressure (p.sub.3), as well as the braking performance assume a maximum.
The braking arrangement according to the invention comprises an adder to which the charging pressure and the exhaust gas counterpressure can be supplied as input signals. A signal can be generated which represents the total pressure (p.sub.total) as the sum of the charging pressure (p.sub.2S) and the exhaust gas counterpressure (p.sub.3). The signal representing the total pressure (p.sub.total) can be supplied to a comparator in which the total pressure (p.sub.total) can be compared with a maximal pressure (p.sub.max) and a signal can be generated which represents a differential pressure (.DELTA.p) of the total pressure (p.sub.total) and the maximal pressure (p.sub.max). The signal representing the differential pressure (.DELTA.p) can be supplied to the brake regulator for generating the actuating signal (S.sub.St).
The new process uses as a control criterion the sum of the charging pressure in the intake pipes and the exhaust gas counterpressure in the pipe section between the cylinder outlets and the exhaust gas turbocharger. The function of the sum of the charging pressure and the exhaust gas counterpressure extends approximately proportionally to the achievable braking performance so that, in the case of a maximum total pressure (formed of the sum of the charging pressure and the exhaust gas counterpressure), the braking performance maximum can also be found.
The total pressure as the sum of the charging pressure and the exhaust gas counterpressure forms a reliable criterion for determining the maximal braking performance.
As a result, it is possible to adjust a braking performance maximum for any rotational engine speed. A maximal pressure is assigned to each rotational engine speed, at which maximal pressure the braking performance reaches a maximum. During the braking operation, the actual rotational engine speed is detected and the actual total pressure is compared with a maximal pressure expediently for the actual rotational speed point and for certain rotational speed ranges. The turbine geometry is adjusted to such an extent that the total pressure corresponds to the maximal pressure. As soon as this is so, the actual total pressure will be at the maximally achievable value. Simultaneously, the braking performance maximum is reached for the actual value of the rotational speed or for the actual rotational speed range.
The maximal pressure can be determined ahead of time or can be determined in the engine running operation.
The maximal pressure can be determined ahead of time. Here, a reference motor is measured over the required rotational speed spectrum and a rotational-speed-dependent pressure curve, which represents the maximal pressure of the sum of the charging pressure and the exhaust gas counterpressure, is established and stored. The pressure curve for the maximal pressure can be used in the running engine operation in order to be able to make a rotational-speed-related comparison between the total pressure (as the sum of the measured charging pressure and the exhaust gas counterpressure) and the maximal pressure.
As a function of the deviation of the total pressure from the maximal pressure, actuating signals are generated in the braking arrangement. These actuating signals control the adjusting device of the turbine geometry. The position of the turbine geometry is changed until the total pressure and the maximal pressure correspond to one another. This construction has the advantage that the maximal pressure for all rotational speed ranges is known ahead of time and the process for adjusting the braking performance maximum can be carried out rapidly.
However, the maximal pressure can also be determined in the running engine operation. Here, the position of the turbine geometry is varied until the total pressure reaches a maximum. The determination of the maximum takes place mathematically in a computer unit of the braking arrangement. This construction has the advantage that the braking performance maximum can be adjusted very precisely because tolerances and spreads are compensated in the case of mass-produced engines.
According to another embodiment, a rotational-speed-dependent characteristic diagram is determined ahead of time for those positions of the guide blades of the turbine geometry for which a braking performance maximum occurs. A defined position of the turbine geometry is assigned to each rotational engine speed, which position corresponds to the braking performance maximum at this rotational speed. This construction has the advantage that the only required information for adjusting the braking performance maximum is the rotational engine speed. The measuring of the charging pressure and of the exhaust gas counterpressure can optionally be eliminated.
Mixed forms of the various embodiments are also contemplated. Thus, it may, for example, be expedient to determine the maximal pressure ahead of time and supply it to the braking arrangement as a rough value which is corrected in the running operation by way of a mathematical determination of the maximum of the measured charging pressure and exhaust gas counterpressure.
On the basis of the rotational engine speed, total pressure and maximal pressure information, the braking arrangement according to the invention generates those actuating signals which are required for an adjustment of the turbine geometry for reaching the maximal braking performance.